1. Field of the Invention
The present invention relates to a method for operating an internal combustion engine, in which a crankcase of the internal combustion engine has a vent into the intake section of the internal combustion engine.
2. Description of Related Art
The crankcase of an internal combustion engine is not permanently sealed. A small amount of gas flows past the piston rings and, without an opening in the crankcase, would create a high pressure therein. This pressure buildup is avoided by way of the so-called crankcase vent. The crankcase vent directs gases out of the crankcase into the intake duct and thus into the intake section of the internal combustion engine. A direct connection to intake duct vacuum is avoided, since otherwise the crankcase would be brought to the pressure level of the intake duct.
Not only are combusted “blow-by” gases pushed via the piston rings into the crankcase, but also, especially when the engine is cold, fuel is introduced into the engine oil that is present, for example, in an oil sump or the like in the crankcase. This fuel is, as a rule, excess injected fuel that does not participate in combustion but instead, for example, condenses on the cylinder walls or is not even vaporized.
This fuel introduced into the crankcase is at first, when the engine is cold, washed into the engine oil. As the engine oil heats up, this fuel evaporates. The vapors then flow through the crankcase vent into the intake duct and then into the combustion chambers. This additional fuel stream results in richer engine operation. Especially at idle, the fuel stream can account for a very large portion of the idle fuel consumption of the internal combustion engine. In contrast to the fuel vapor stream from the tank vent, the fuel vapor steam through the crankcase vent is generally not controllable. In other words, a crankcase ventilation valve that is, for example, electrically controllable generally is not present.
The problem of outgassing from the engine oil is very pronounced with fuel having a high ethanol content. Such fuels are increasingly being used for the operation of vehicles having a spark-ignited engine. Such vehicles are called “flex-fuel” vehicles (FFVs). Pure ethanol is called E100, pure gasoline E0, and any mixture is referred to as EX. In Europe and the United States, usual ethanol-containing fuels contain approximately 75 to 85% ethanol (E85 blend). The remainder (15 to 25%) is gasoline.
It is known that ethanol is more difficult to vaporize at low temperatures than pure gasoline. When E85 is used, considerably more fuel therefore needs to be injected for a cold start than in the case of pure gasoline. The result of this is that during a cold start and in the warmup phase, considerably more fuel is scraped off by the piston rings into the engine oil than in the case of operation with pure gasoline. Because ethanol is a pure substance having a fixed boiling point (78° C.), this added fuel then outgases quite abruptly as the oil heats up. At low load speeds and in particularly at idle, this results in a significant enrichment that cannot always be managed with the lambda control system alone. Especially when frequent extreme cold starts are performed without allowing the engine oil to heat up, a great deal of fuel can collect in the engine oil. Levels of 200 to 500 ml of fuel in the engine oil have been routinely observed.
If the internal combustion engine is warmed up in a short period of time (e.g. using heavy load and at high speed) with such a large input of fuel into the engine oil, and is then operated at idle, the volumetric flow of the exhaust gas often cannot be managed. Further leaning by way of the injection valves is then often impossible, since the minimum injection time has already been reached. In addition, a maximum fuel proportion of, for example, 60% must not be exceeded.